Reflector antennas with self-supported feeds are chiefly used because they are straightforward and inexpensive to manufacture. They also provide higher antenna efficiency and lower side lobes in the radiation pattern than is the case when the feed has to be supported by diagonal struts. The drawback with the latter configuration is that the main reflector becomes blocked by the struts. A self-supported feed is also easily accessible from the back of the reflector, thus is frequently selected when it is best to locate the transmitter and/or the receiver there. This also reduces the loss that otherwise occurs when the electromagnetic waves have to be led in a cable along one of the support struts.
The European Patent EP 87903452.8 publ no 0268635, U.S. Pat. No. 4,963,878 and U.S. Pat. No. 6,020,859 describe different versions of reflectors with self-supported feeds, where the feed consists of a waveguide tube, a dielectric joint and a sub-reflector. The tube is attached to the center of the rotationally symmetric reflector and extends to the focal region of it. The sub-reflector is located in front of the tube, and the surface of this sub-reflector is provided with rotationally symmetric grooves also called corrugations. By these means the electromagnetic waves are prohibited from propagating along the sub-reflector surface independent of whether the electric field is normal to the surface or is tangential to it. The result is that the radiation pattern has higher directivity, lower spillover and lower far out sidelobes than otherwise would be possible.
The present invention relates to several improvements of the antennas described in European Patent EP 87903452.8 publ no 0268635, U.S. Pat. No. 4,963,878 and U.S. Pat. No. 6,020.859. The improvements are for improved readability in the below description denoted: ring focus reflector, elevated central region, metal screws, rim corrugations, simple tube dual band and feed protection.
Ring Focus Reflector
The antennas described in the above referenced European and U.S. patents and U.S. patent application make use of a main reflector which is rotationally symmetric and has a substantial parabolic shape. However, the antenna will have higher gain if the main reflector shape is improved. The present invention describes how to improve the shape of the main reflector.
Elevated Central Region
It is not possible to design the antennas in the above referenced European and U.S. patents and U.S. patent application with low reflection coefficient at the waveguide input. The reason for this is reflections from the region around the tube in the center of the main reflector. In the improvement of the antenna this problem is solved by modifying the reflector in its central region.
Metal Screws
In the above referenced European and U.S. patents and the U.S. patent application the sub-reflector is supported to the end of the waveguide tube by means of a dielectric joint, which partly or totally fills the gap between the sub-reflector and waveguide tube end, and which is interlocked with and glued to the sub-reflector and waveguide tube end. This gluing does not provide a sufficiently strong mechanical support in all applications. In the present invention this is improved for linearly polarized applications by means of metal screws or thin cylinders or plates which provide a strong metal connection between the sub-reflector and the end of the tube.
Rim Corrugations
In the above referenced European and U.S. patents and the U.S. patent application there will be large back-lobes in the direction opposite to the main lobe. The invention reduces these lobes by means of one or more corrugations or grooves or metalized dielectric rings around (or in the structure behind) the rim of the reflector.
Simple Tube
In the previous embodiments of the referenced European and U.S. patents the waveguide support tube has an inner diameter which changes near the end of the tube which is closer to the sub-reflector, and in some cases it was also necessary to insert one or more irises into this end of the tube, all in order to properly match the antenna to obtain a low reflection coefficient. The present invention describes an improvement by which the waveguide tube can be a circular cylindrical tube of constant cross-section along its length. This improvement significantly reduces manufacturing cost.
Dual Band
In the above referenced European and U.S. patents and the U.S. patent application, the antenna is fed through a circular waveguide for operation in a single frequency band of up to 20% bandwidth. In some applications dual band operation is of interest, e.g. one band for transmission and another for reception of signals. The invention describes a modified antenna which is fed by two waveguides; one inner circular waveguide and outside this a coaxial waveguide.
Feed Protection
In some applications the antenna may be located in hostile environments, and water, dust and other undesired material may gather in the region between the end of the tube and the sub-reflector and thereby destroy the performance. The present invention describes how the antenna in the above referenced European patent can be improved to be less sensitive to such effects.